Ixodes hexagonus is the main candidate as vector of Theileria annae in northwest Spain.

Babesia canis and Babesia gibsoni have, until recently, been considered the only piroplasms that parasitise dogs. However, recent reports indicate that "small" Babesia infections in Spanish dogs are surprisingly frequent and molecular phylogenetic analysis indicates that the infecting agent is closely related to Babesia microti. Because the 18SrDNA sequence was not completely identical to that of B. microti, the new name "Theileria annae" was assigned to the canine agent. No information is available regarding the possible vector of the new piroplasm, T. annae. As part of an effort to identify the tick that may transmit T. annae in northwest Spain we asked veterinary surgeons practising in the region to collect and send to our laboratory ticks from dogs visiting their clinics. Seven hundred and twenty ticks collected from dogs of unknown clinical status during 1998 and 636 ticks collected between November 2001 and March 2002 from 38 dogs infected with T. annae and 131 uninfected dogs were identified. Results from the first study indicated that among the Ixodidae, Ixodes hexagonus clearly predominates over Ixodes ricinus (26.11% versus 6.67%). This observation was consistent with results of the second study, in which I. hexagonus was detected in all infected dogs and 71.8% of non-infected dogs and I. ricinus was not detected in either the infected or non-infected dogs. Results from the 2001-2002 study also indicate that the presence of Dermacentor reticulatus adult females is significantly less frequent among infected than non-infected dogs (OR=0.44; 95% CI: 0.21-0.92). On the other hand, I. hexagonus adult females and males are 6.75 and 4.24 times more likely to be detected among infected than non-infected dogs, respectively, with the association being, in both cases, statistically significant (95% CI: 1.97-23.12 and 1.92-9.36, respectively). I. hexagonus emerges as the main candidate as vector of T. annae because it feeds on dogs more frequently than other ticks and because B. microti is transmitted by Ixodes ticks, both in North America and Europe. In the absence of definitive confirmation of this hypothesis, our observations suggest that I. hexagonus might serve the same role as does Ixodes scapularis (=Ixodes dammini), the vector of B. microti in eastern North America.